The present invention relates to disturbance signal recorders, also called fault recorders. These devices have two characteristic operating modes, namely a watching mode during which only electrical input signals are monitored, and a recording mode during which the electrical input signals are effectively recorded. The start of recording is generally a few tenths of a second before the appearance of the disturbance or trouble in order to subsequently make it possible to obtain details of the complete history thereof.
There are two main types of disturbance signal recorders, namely electromechanical and electronic recorders, but the present invention relates to the latter type.
According to a first known embodiment falling in this category, an electronic device producing a time lag or delay is inserted upstream of the device for writing on paper, which uses optical galvanometers, a halogen lamp and a photosensitive paper. For this purpose electrical input signals, after undergoing analog-digital conversion, pass through the delay device or pretriggering memory or store formed with the aid of a shift register and are then reconverted into analog signals for application to the galvanometers. This solution has a number of disadvantages due essentially to the writing devices used. Galvanometers with halogen lamps are expensive and the fact that they are constantly supplied with power is prejudicial to their reliability. In addition, the need to periodically replace the roll of paper used leads to high operating costs. Finally, the photosensitive paper of the writing or recording device has a poor storage life, both before and after use.
According to a second known solution, certain of these disadvantages are obviated by using a conventional writing or recording device having styli, so that costs are reduced and the paper has a good life. However, the stylus recorder has a reduced pass band of approximately 30 Hz at the most and for reproducing signals with a higher frequency a device is provided for slowing down the signals to make them compatible with the stylus recorder. This device is constituted by a solid state electronic memory, which is inserted between the shift register, i.e. the pretriggering memory, and digital-analog reconversion circuits. A disadvantage of the stylus recording device is the difficulty of reproducing in a simple manner and at the same time as the input signals, the date information necessary for processing the recordings.
In addition, these two solutions have two important deficiencies. The first is the need of systematically replacing the roll of paper and the second is the lack of flexibility of the information support, a diagram on paper not really being suitable for automatic analysis and digital processing without the use of a human operator.
A third solution obviating the aforementioned difficulties is described in French Patent Application 2,429,998. The useful input signals are recorded on a flexible disk magnetic memory inserted between the shift register constituting the so-called pretriggering memory and the digital-analog conversion circuits preceding a recorder on paper. The capacity of the disk memory is equivalent to several rolls of paper. Thus, the size of this memory makes it possible to considerably reduce the frequency with which the rolls of paper are replaced. If the writing or recording device is detachable, the operator can transport it with him and reproduce on the paper the recordings of the day of his or her visit.
The disadvantages of this latter electronic solution are mainly in the type of memory used, the latter forming an integral part of the recording chain. For certain applications, such as the monitoring of electric power transmission lines in tropical countries, the characteristics of the flexible disks (temperature range for use 5.degree. to 45.degree. C., permitted relative humidity 20 to 80%) may prove inadequate. Moreover, a breakdown in the flexible disk memory automatically leads to a failure of the recorder which, from then on, is no longer able to collect new information.